1. Field of the invention
The present invention relates to the integration of GaAs on a Si substrate.
The invention more specifically relates to the monolithic integration of GaAs/AlGaAs electronic or optoelectronic structures such as light emitting diodes on a Si substrate on which Si electronic circuitry such as LED-drivers (field effect transistors) can also be integrated.
2. Description of the prior art ( GaAs layers or GaAs structures, i.e. GaAs/AlGaAs light emitting diodes can be grown substantially free of stresses on top of a Si substrate, usually in wafer form, at high temperature, i.e. 700.degree. C.
However, during the cooling-off period the different shrinkage of GaAs and Si brings about stresses in the materials.
These stresses may cause the wafer to bend if the thickness of the GaAs layer grown on the Si substrate is rather small, e.g. when this thickness does not exceed 3 micrometer.
If the GaAs layer is thicker, microcracks might appear in the epitaxial layer structure on the wafer.
The presence of material stresses affects negatively the electrical and/or optical properties of the GaAs/Si devices. Material warpage hinders lithography and further processing of the wafer. The randomly occurring microcracks might also interfere with the proper operation of the electronic devices that are ultimately formed.
The above issues of concern that arise when growing GaAs layers on Si substrates are described in the following publications: "Stress variations due to microcracks in GaAs grown on Si" published by B.G. Yacobi et al. in Appl. Phys. Lett. Vol. 51, No. 26 and "Stress variations and relief in patterned GaAs grown on mismatched substrates" by B.G. Yacobi et al. in Appl. Phys. Lett. Vol. 52, No. 7.
Few solutions have been proposed in the art to deal with the above problems. In Appl. Phys. Lett., Vol. 53, No. 3 a paper on "The warpage of GaAs-on-Si wafers and its reduction by selective growth of GaAs through a silicon shadow mask by molecular beam epitaxy" was published. The warpage of fully covered GaAs-Si wafers and of Si wafers with selective growth of GaAs (obtained by using a Si shadow mask) have been studied. A significant reduction in warpage is observed by restricting the growth area of GaAs.
It is also known to provide the back side of the silicium substrate with a film in order to counteract the stresses that are induced by the GaAs epitaxial layer.